The evolution of the central nervous system in mammals was a natural response to an increasingly complex environment requiring solutions to difficult problems. The resulting structure is an intricate biochemical matrix that is precisely controlled and attenuated through an elaborate system of chemically modulated regulatory pathways. Through an elaborate series of highly specific chemical reactions, these pathways oversee and direct every structural and operational aspect of the central nervous system and, through it, the organism itself Normally the complex interplay of the various control systems cooperates to produce a highly efficient, versatile central nervous system managed by the brain. Unfortunately, when the biochemical matrix of the central nervous system is damaged, either through age, disease or other reasons, the normal regulatory pathways may be incapable of effectively compensating for the loss. In such cases it would be highly desirable to modify or supplement the neural mechanisms to prevent or compensate for such disorders. That is the focus of the present invention.
More specifically, the mammalian brain is composed of approximately ten billion nerve cells or "neurons" surrounded by a even greater number of support cells known as neuroglia or astrocyte cells. Neurons, like other cells of the body, are composed of a nucleus, a cytoplasm and a surrounding cell membrane. However, unlike other cells, neurons also possess unique, fiberlike extensions allowing each individual nerve cell to be networked with literally thousands of other nerve cells to establish a neural infrastructure or network. Communication within this intricate network provides the basis for all mental processes undertaken by an organism.
In each nerve cell, incoming signals are received by neural extensions known as "dendrites" which may number several thousand per nerve cell. Similarly, neural information is projected along nerve cell "axons" which may branch into as many as 10,000 different nerve endings. Together, these nerve cell axons and dendrites are generally termed "neurites" through which each individual neuron can form a multitude of connections with other neurons. As a result, the number of possible neural connections in a healthy brain is in the trillions, giving rise to tremendous mental capacity. Conversely, when the connections within the neural network break down as nerve cells die or degenerate due to age, disease or direct physical insult, the mental capacity of the organism can be severely compromised.
The connection of the individual axons with the dendrites or cell bodies of other neurons takes place at junctions or sites known as "synapses." It is at the synapse that the individual neurons communicate with each other through the flow of chemical messengers across the synaptic junction. The majority of these chemical messengers, or "neurotransmitters," are small peptides, catecholamines or amino acids. When the appropriate stimulus is received by a neural axon connection, the neurotransmitters diff-use across the synapse to the adjacent neuron, thereby conveying the stimulus to the next neuron along the neural network. Based upon the complexity of the information transferred between the nerve cells, it is currently believed that between 50 and 100 distinct neurotransmitters are used to transmit signals in the mammalian brain.
Quite recently, it was discovered that nitric oxide (NO) and carbon monoxide (CO) may function as neurotransmitters. These gaseous molecules appear to participate in a number of neuronal regulatory pathways affecting cell growth and interactions. In the brain, as well as in other parts of the body, CO is produced by the enzyme "heme oxygenase II" (HO). Whether produced from the HO enzyme or from other sources, it is believed that when CO diffuses into a neuron it induces a rise in a secondary transmitter molecule known as "cyclic guanosine monophosphate" (cGMP), by modulating an enzyme known as "guanylate cyclase" or "guanylyl" cyclase. Thus, CO acts as a signaling molecule in the guanylyl cyclase regulatory pathway. The resultant increase in cGMP levels appears to modify several neurotropic factors as well as other neuronal factors which may induce, promote or modify a variety of cellular functions including cell growth and intercellular communication.
Neurotrophic factors are molecules that exert a variety of actions stimulating both the development and differentiation of neurons and the maintenance of cellular integrity and are required for the survival and development of neurons throughout the organism's life cycle. Generally, neurotrophic factors may be divided into two broad classes: neurotrophins and pleiotrophins. Pleiotrophins differ from the neurotrophins in that they lack a molecular signal sequence characteristic of molecules that are secreted from cells and they also affect many types of cells including neurons. Two effects of neurotrophic factors are particularly important: (i) the prevention of neuronal death and (ii) the stimulation of the outgrowth of neurites (either nascent axons or dendrites). In addition, it appears that CO-induced neurotrophic factors may reduce the membrane potential of nerve cells making it easier for the neurons to receive and transmit signals.
Many of today's researchers believe that memory is associated with the modification of synaptic activity, wherein the synaptic connections between particular groups of brain neurons become strengthened or facilitated after repeated activation. As a result, these modified connections activate much easier. This type of facilitation is believed to occur throughout the brain but may be particularly prominent in the hippocampus, a brain region which is crucial for memory. The stimulation of neuronal pathways within the hippocampus can produce enhanced synaptic transmission through these pathways for many days following the original stimulation. This process is known as long term potentiation (LTP).
More particularly, long term potentiation is a form of activity-dependent synaptic electrical activity that is exhibited by many neuronal pathways. In this state, generally accepted as a type of cellular memory, nerve cells are more responsive to stimulation. Accordingly, it is widely believed that LTP provides an excellent model for understanding the cellular and molecular basis of synaptic plasticity of the type that underlies learning and memory in vertebrates, including man.
NO and CO are currently the leading candidates for messenger substances that facilitate LTP because inhibitors of these compounds retard the induction of potentiation. The ability to modify neural activity and to increase the ease of LTP using these or other signal transducers could potentially increase learning rates and cognitive powers, possibly compensating for decreased mental acuity. Prior to the present invention, there were no known agents which could operate on the cellular level int vivo to reliably modify neural regulatory pathways so as to facilitate the LTP of neurons.
In contrast to the enhanced mental capacity provided by long term potentiation, mental functions may be impeded to varying degrees when the neuronal network is disrupted through the death or dysfunction of constituent nerve cells. While the decline in mental abilities is directly related to the disruption of the neural network, it is important to remember that the disruption is occurring on an individual cellular level. At this level the deleterious effects associated with neuronal disruption may be brought about by any one of a number of factors including neurodegenerative diseases and disorders, aging, trauma, and exposure to harmful chemical or environmental agents.
Among the known neurological diseases which adversely impact neuronal function are Alzheimer's disease and related disorders, Parkinson's disease, motor neuropathic diseases such as Amyotrophic Lateral Sclerosis, cerebral palsy, multiple sclerosis, and Huntington's disease. Similar problems may be brought about by loss of neuronal connectivity due to normal aging or through damage to neurons from stroke or other circulatory complications. Direct physical trauma or environmental factors including chemical agents, heavy metals and the like may also provoke neuronal dysfunction.
Whatever the cause of the neural disorder or dysfunction, the general inability of damaged nerve cells to undergo substantial regrowth or regeneration under natural conditions has led to the proposal that neurotrophic factors be administered to nerve cells in order to help restore neuronal function by stimulating nerve growth and function. Similarly, stimulating neuritogenesis, or the growth of neurites, by administering neurotrophic factors may contribute to the ability of surviving neurons to form collateral connections and thereby restore neural function.
At present, prior art techniques and compounds have not been effective or practical to directly administer neurotrophic factors to a patient suffering from a neural disorder. In part, this is due to the complex molecular interaction of the neurotrophic factors themselves and to the synergistic regulation of neural cell growth and neuritogenesis. Neurotrophic factors are the result of a long chemical cascade which is exquisitely regulated on the molecular level by an intricate series of transmitters and receptors. Accordingly, neuronal cells are influenced by a concert of different neurotrophic factors, each contributing to different aspects of neuronal development at different times. Neurotrophic factors are, effectively, the tail end of this cascade and thus are one of the most complex components of the regulatory pathway. As such, it was naive for prior art practitioners to assume that the unattenuated administration of single neurotrophic factors at random times (from the cells viewpoint) could substantially improve cell activity or regeneration. In contrast, modification of the regulatory pathway earlier in the cascade could allow the proper growth factors to be produced in the correct relative amounts and introduced into the complex cellular environment at the appropriate time.
Other practical considerations also preclude the prior art use of neurotrophic factors to stimulate the regeneration of the neuronal network. Neurotrophic factors (including neurotrophins and pleiotrophins) are large proteins and, as such, are not amenable to normal routes of medical administration. For example, these proteins cannot be delivered to a patient or subject orally as the patient's digestive system would digest them before they reached the target neural site. Moreover, due to their relatively large size, the proteins cannot cross the blood brain barrier and access the most important neurological site in the body. Alternatively, the direct injection of neurotrophic factors into the brain or cerebrospinal fluid crudely overcomes this difficulty but is fraught with technical problems of its own which have thus far proven intractable. For example, direct infusion of known neurotrophins into the brain has proven impractical as it requires administration over a period of years to provide therapeutic concentrations. Further, direct injection into the brain has been associated with dangerous swelling and inflammation of the nerve tissue after a very short period of time. Thus, as theoretically desirable as the direct administration of neurotrophic factors to a patient may be, at the present time, it is unfeasible.
Accordingly, it is a general object of the present invention to provide methods and associated compositions for effectively modifying mammalian neurons or neural activity to achieve a variety of beneficial results.
Thus, it is another object of the present invention to provide methods and associated compositions for treating mammalian neurological diseases and disorders.
It is yet another object of the present invention to provide methods and associated compositions for inducing long term changes in the membrane potential of a mammalian neuron.
It is still yet another object of the present invention to provide methods and associated compositions for inducing the iii vivo physiological production of genetically encoded molecules and neurotrophic factors within cells.
It is a further object of the present invention to provide methods and associated compositions for enhancing the neurotogenic effects of neurotrophic factors in a physiological environment.